For a steering device of a vehicle, an electric power steering (EPS) apparatus is conventionally known that applies a steering assist force on a steering mechanism by driving an electric motor in accordance with a steering force exerted by a driver who steers a steering wheel.
The electric power steering apparatus includes a reverse connection protecting device configured to protect a controller (ECU: Electronic Control Unit) from an overcurrent when a battery (rechargeable battery) is reversely connected. The reverse connection means a connection in which a positive pole and a negative pole of the battery are connected reversely with respect to normal connection for controller ECU). The reverse connection protecting device includes an in-vehicle relay and a driving circuit thereof. The in-vehicle relay incorporates a contact-type mechanical switch and an electromagnetic coil, and an excitation current flowing through the electromagnetic coil makes the coil an electromagnet thus attracting an iron piece of the switch, bringing movable and fixed contacts into contact, and turning on the switch. When the excitation current is no longer flowed to the electromagnetic coil, the movable and fixed contacts are separated by a return spring provided on the iron piece of the switch. The reverse connection protecting device is a typical method of protecting the reverse connection since the switch of the in-vehicle relay is not turned on in the reverse connection.
However, the in-vehicle relay used for the reverse connection protecting device is for large currents, and thus is of large scale compared to other elements, and is also expensive. With reduction in the price of vehicles in recent years, demands to reduce the price of the electric power steering apparatus is also becoming stronger, and the electric power steering apparatus of a type that does not include the reverse connection protecting device is being demanded as part of the price reduction. In the general electric power steering apparatus of a type that does not include the reverse connection protecting device, a fuse arranged in the vehicle is blown when the battery and the controller (ECU) are reverse connected, thus preventing smoking and ignition of the controller (ECU). For example, in an abnormality history retention device described in PTL 1, an abnormality of a secondary battery is detected based on an output of the secondary battery and the detection result is output, and when the switching element permits electric conduction depending on the detection result, a discharging current flows from the secondary battery thus causing heat generation in a heat generating element, and a temperature fuse is blown by the heat from the heat generating element.